THE BINOCULAR MICROSCOPE. 431 



Similar results were obtained when the powers were further 

 increased to 76, 73, and 71 diameters respectively, with the 

 exception that wdth the Wenham there was less falling off in the 

 relief. It is remarkable that the observed difference in the stereo- 

 scopic relief depends to some extent upon the objects on the stage. 

 For example, if the objects are the inside and outside views of 

 Triceratium fimbriatum there will be no very great difference in 

 the stereoscopic relief in these three instruments, but let this 

 object be exchanged for some polycistines, a considerable difference 

 in the strength of the stereoscopic effects will at once be apparent. 

 It is therefore most important that anyone, when comparing the 

 stereoscopic relief in different kinds of binoculars, should take care 

 to have a suitable object upon the stage. 



There is another interesting point worth notice, viz. if an object 

 under a Greenough be examined with only one eye, and a mental 

 estimate of its size be made, then the eye removed from the 

 instrument and distant objects about the room looked at for a 

 few seconds, afterwards the object under the microscope again 

 examined, but this time with both eyes, the image will appear 

 smaller than when seen by only one eye. With the Wenham 

 there is not this difference, but with the Parallel tube instrument 

 the difference is the other way, the image seen with both eyes 

 being larger than that seen with one eye. It is necessary to focus 

 the eyes upon things in the room before each separate experiment 

 is made. 



As to the ease and comfort of vision, to my eyes the Greenough 

 and Wenham are similar, while that in the Parallel tube is not 

 quite so comfortable, when it is arranged for ortho-stereoscopic 

 images, because of the necessity of keeping the head perfectly 

 steady, so that the emergent pencils may be equally cut by each 

 eye. This discomfort increases with the higher powers, as the 

 emergent pencils get smaller. This certainly is the defect in all 

 those kinds of instruments where the stereoscopic relief depends 

 upon the cutting of the emergent pencils by the iris of the eyes. 

 It is not a very serious one ; if caps with stops cutting out about 

 one- quarter of the aperture with low eye- pieces, and about one- 

 third w^ith high ones, are placed over the eye-pieces, vision is 

 thought to be easier. This plan of using caps with stops over 



